1. Field of the Invention
This invention relates to sealed shaft bearing assemblies and more particularly to an improved sealed bearing assembly employing a seal wear ring which substantially reduces or eliminates both journal fretting and cone back face wear.
2. Description of the Prior Art
Roller bearing assemblies incorporating two rows of tapered roller bearings preassembled into a self-contained package for mounting onto journals at the ends of axles or shafts are well known and used in a variety of applications. Since these bearings are widely used as rail car bearings assembled onto journals at the ends of rail car axles, the present invention will be described with reference to such rail car bearings, it being understood that the bearings may be employed on shaft journals for various other uses. In bearings of this type, the two rows of tapered roller bearing elements are fitted one into an outer race at each end of a common bearing cup, and a pair of bearing cones defining the inner races normally have an inner diameter dimension to provide an interference fit with the shaft journal. A cylindrical sleeve or spacer positioned between the cones provide an accurate spacing of the inner races on the journal. Seals mounted within each end of the bearing cup provide sealing contact with wear rings positioned one against the outer end faces of the bearing cones at each end of the assembly. The entire assembly is prelubricated and adapted to be pressed as a unit onto the end of the shaft journal.
In a typical rail car installation, the axle journal is machined with a fillet at the inboard end of the journal, and a backing ring is machined to accurately fit the contour of the fillet and engage the inboard end of the inner wear ring to accurately position the bearing assembly on the axle. An end cap mounted on the end of the axle by bolts threaded into bores into the axle engages the outboard wear ring and clamps the entire assembly on the end of the axle and applies an axial compressive load to the assembly between the fillet and the end cap. Examples of such bearings may be found, for example, in U.S. Pat. Nos. 3,494,682 and 5,380,102.
Bearings of the type described above are frequently subjected to high static and dynamic loading which results in flexure of the journal which in turn results in relative movement between the journal surface and the wear ring fitted thereon, and between the end face of the wear ring and the abutting end surface of the inboard bearing cone. As is well known, this relative movement can, over time, produce fretting resulting in a groove in the journal surface and an enlargement of the wear ring bore, and wear between the end of the wear ring and bearing cone may produce a groove in the cone face. This can result in the wear ring becoming so loose as not to maintain the desired axial clamping force on the assembly, or in the wear ring becoming so loose as not to remain perfectly concentric with the journal surface which, in turn, may ultimately produce seal failure.
One proposal to solve the journal fretting and end face wear problems is illustrated in U.S. Pat. No. 5,017,025 which discloses use of a seal wear ring having an internal diameter greater than the diameter of the journal to provide a space between the wear ring and the journal throughout the length of the wear ring. The backing ring is provided with a counterbore and a shoulder dimensioned to receive the end of the wear ring, with an inference fit, to align the wear ring relative to the axis of rotation. It has been found, however, that it is not always possible to obtain precise axial alignment when the bearing is installed, and further that when pressing the bearing onto the shaft, the end of the wear ring is not always precisely aligned with the counterbore and the backing ring so that the heavy pressing loads can mar the backing ring and/or shoulder which, in turn, can result in further misalignment. Even though journal fretting is eliminated by the absence of contact between the journal surface and wear ring, any misalignment can lead to excessive seal wear and ultimate failure, and to accelerated end face wear between the seal wear ring and bearing cone.
Occasionally during installation or removal of a bearing assembly, the weight of the backing ring and the interfitting wear ring will cover the inboard wear ring/backing ring assembly to fall out of the bearing. This can result in the loss and/or contamination of lubricant.
In older bearing designs, fretting was also common between the backing ring curved internal surface and the journal fillet surface since these parts moved relative to each other as a result of journal bending. In a railcar bearing installation, component compression at the 6 o'clock position caused the backing ring to move "up" or radially outward along the journal fillet surface. Conversely, release of the clamping load at the 12 o'clock position caused the backing ring to move "down" or radially inward and, of course, journal rotation continuously revolved these positions around the fillet, resulting in fretting around the periphery of the contacting curved surfaces. This problem was overcome by a change of the backing ring design which involved providing an axially extending flange which telescoped over a portion of the axle adjacent the maximum diameter portion of the fillet, with an interference fit sufficient to provide a preload or tensile stress in the backing ring during installation, which tensile stress must be relieved before compression at the 6 o'clock position (resulting from journal bending) can produce relative movement resulting in fretting. While this essentially eliminated the backing ring--fillet fretting problem, relative movement and fretting between the wear ring and the journal surface and between the wear ring and abutting cone face have not been previously successfully addressed.
U.S. Pat. No. 5,549,395 also illustrates a bearing arrangement in which the wear ring inner cylindrical surface is spaced from the journal surface along the full wear ring length. In this case, the coaxial relation between the wear ring and journal surface is maintained by a low friction dimensionally stable plastic ring interposed between the journal and wear ring inner surface. While this arrangement prolongs seal life, end face fretting is not entirely overcome.
U.S. Pat. No. 5,462,367 illustrates another arrangement for overcoming the problem of journal fretting by eliminating the seal wear ring entirely and providing an axial extension on the bearing cones which contact the resilient seal. This arrangement provides coaxial relation between the seal and the cylindrical sealing surface on the cone extension, but inherently requires the seal to be placed in close relation to the rolling elements. Pumping action of the rolling bearings, at high speed, and turbulence resulting from impact loadings and the like can result in undesired seal leakage.
It is also known that the interference fit between the wear ring and the recess in the backing ring results in little if any fretting at the interface between the wear ring and backing ring.
While the interference fit between the wear ring and the backing ring recess theoretically provides a coaxial relation between the wear ring outer surface and the journal axis, under no load or low load condition, journal bending can result in the sealing surface of the wear ring not being concentric with the journal at the point of seal contact. Thus, it has normally been considered desirable for the wear ring to contact the journal surface at least adjacent the bearing cones to provide a sealing surface that is concentric to the journal as shown, for example, in U.S. Pat. No. 3,494,682, mentioned above. The wear ring is dimensioned to provide an interference fit with the journal and this interference fit normally substantially eliminates journal fretting under light loads; however, under heavy loads encountered on today's rail cars, fretting still occurs between the wear ring and journal surface and also between the bearing cone end face and the end of the wear ring. Accordingly, it is an object of the present invention to provide a bearing design which includes a seal wear ring and which eliminates fretting between the wear ring and the journal surface and which substantially eliminates fretting between the wear ring and the bearing cone end face.
Another object of the invention is to provide such a bearing in which direct contact between the wear ring and the journal surface is avoided while maintaining the desired concentric relation between the wear ring surface and the journal axis.
Another object of the invention is to provide such a bearing including a seal wear ring which is maintained concentric with the shaft journal at each end of the wear ring without the necessity of contact with the journal surface.
Another object is to provide a preassembled, pre-lubricated bearing assembly in which the wear ring/backing ring subassembly is captured and cannot fall out of the bearing assembly.